Process for degreasing and oxalating in a single spray stage



United States Patent 3,116,177 PROCESS FGR DEGREASING AND OXALATING IN ASINGLE SPRAY STAGE Paolo de Cerrna and Roberto Vironda, Milan, Italy,

assignors to Montecatini Societa Geuerale per lIndustria Mineraria eChimica, Milan, Italy No Drawing. Filed (Bet. 17, 196i), Ser. No. 62,903Claims priority, application Italy Oct. 19, 1959 3 Claims. (Cl. 148-614)This invention relates to an improvement in a process for pre-treating ametal surface, particularly iron, or steel, or other ferrous metal oralloy, prior to applying paint thereto.

Conventional continuous or cyclic processes of this nature requirepreliminary degreasing of the surface of the metal, prior to treatmentwith a spray to form the coating which comprises either a phosphate oroxalate layer, or both.

The first step of such prior processes comprises a spray degreasing bymeans of alkali, or an organic solvent emulsion. This is followed by awashing step, and successively, in a third step, by treatment withphosphate or oxalate bath. The steps of washing and passivation thenfollow. This multistage process requires a complicated plant, andinvolves high operating costs.

Unsuccessful attempts have been made to combine degreasing andphosphatation in a single stage, by adding an emulsion of organicsolvents to the phosphate baths. However, the results have never beensatisfactory because the phosphate layer formation is rather irregular,especially after a certain period of pollution of the bath. Thisdisadvantage is due to the crystalline structure of the phosphate layer,which absorbs oils and fats with great facility. In consequence, thesurface is more or less spotted after the treatment, due to the tracesof oil or fat absorbed.

Reports are found in the technical literature of the theoreticalpossibility of combining the degreasing and oxalation in a single stageor phase. Nevertheless, the reports clearly specify that the results areinferior to those obtained by a cyclic process in which degreasing andoxalation are separate operations.

Surprisingly, the applicants have found that by adding to the oxalicacid baths a degreasing substance consisting of an aqueous emulsion oforganic solvents, for example kerosene, stable in acid medium, perfectresults are constantly obtained. This surprising result is due to theamorphous character of the layer of oxalate, which has a tendency toabsorb Water and to reject oils and fats.

Even after advanced ageing of such baths, and even when a considerablelayer of oil or fat removed from the plate floats at the surface, theoxalate layer that results is in perfect formation. The layers areuniform, and free of spots usually originating from absorption of oilsor fats.

Another great advantage of the present invention is the obtaining of asurface which is especially suitable for anchorage of the after-appliedcoat of paint, by the removal of the layer of excess oxalate aftertreating with a diluted chromic acid solution.

In the first phase or stage, the material is degreased and coated with alayer of oxalate, then it is washed and subjected to the action of adiluted chromic acid solution whose concentration is between 0.05 and5%, preferably between 0.5 and 2%.

The chromic acid solution removes a large part of the oxalate layer,leaving a metallic, dark grey colored, surface that is extraordinarilyeffective for anchorage of paint.

After treating with the chromic acid solution, the material is rinsed toremove the chromic acid excess.

3,116,177 Patented Dec. 31, 1963 The invention will be now illustratedby two examples, which are preferred embodiments, but are not intendedto be limitative.

Example I A bath is prepared containing in 1 liter water:

Grams Oxalic acid 15 Sodium tripolyphosphate 1 Ferrous sulphate 1.5Sodium nitrite 1.5

This mixture is sprayed upon a degreased iron plate, to produce oxalatelayers.

By adding, to such bath, 15 grams per liter kerosene emulsion in waterkerosene), oxalate layers are obtained of the same quality as above onsheet iron not previously degreased. The bath can be used until it hastreated 4 m plate per liter of bath, with unvarying results. In thisway, three operating phases are suflicient (degreasing-oxalation,+washing +passivation) to obtain a normal oxalate layer.

Example II Upon employing a bath identical with that described inExample I, but adding 15 grams per liter of an aqueous kerosenesolution, amorphous oxalate layers of the normal type are obtained, in asingle stage treatment. The materials so treated are rinsed in runningwater, subjected to the action of a chromic acid solution, for examplefor one minute and finally again quickly rinsed to remove the excesschromic acid. The surface so obtained is free of any apparent surfacelayer, is uniformly grey colored, and is especially suitable for theanchorage of the successively applied paint. The material treated andpainted in such a manner may even be deformed, without any detachment ofthe paint occurring.

It is noted, that the conventional phosphatation and oxalation slightlyimpair the mechanical anchorage of the paint, so that, in tests on thepainted material, better resistance to deformation is found withuntreated and directly painted material. In contrast, in the case ofmaterial treated according to the herein claimed process, i.e.degreasing-oxalation +washing +treatment with chromic acid -|-summarywashing, the deformation tests always show an advantage over untreatedmaterial. For example, in the embossing test carried out according toErichson, such advantage ranges between 1 and 2 mm, according topainting cycle employed.

We claim:

1. A process of preparing a ferrous metal surface for painting bysimultaneously degreasing and forming an amorphous oxalate layer on saidmetal surface by means of an oxalating bath consisting of oxalic acid ina concentration of about 15 grams per liter, sodium nitrite in aconcentration of about 1.5 grams per liter, a ferrous compound having anFe++ concentration of about 0.6 gram per liter and an acid stableaqueous emulsion of an organic solvent having a concentration of about15 grams per liter, rinsing the amorphous oxalate layer so formed withwater and then subjecting said surface to the action of an aqueouschromic acid solution having a chromic acid concentration between 0.5and 50 grams per liter, and then quickly rinsing oif any excess chromicacid.

2. A process of preparing a ferrous metal surface for painting bysimultaneously degreasing and forming an amorphous oxalate layer on saidmetal surface by means of an oxalating bath consisting of oxalic acid,sodium nitrite, a ferrous compound and an acid stable aqueous emulsionof a hydrocarbon oil, rinsing the amorphous oxalate layer so formed withwater and then subjecting References Cited in the file of this patentUNITED STATES PATENTS 2,744,555 Nicholson et al May 8, 1956 4 PiccinelliJuly 23, 1957 Cerma Dec. 9, 1958 Heller et a1. June 6, 1961 FOREIGNPATENTS Great Britain of 1913 Germany May 9, 1956

1. A PROCESS OF PREPARING A FEROUS METAL SURFACE FOR PAINTING BYSIMULTANEOUSLY DEGREASING AND FORMING AN AMORPHOUS OXALATE LAYER ON SAIDMETAL SURFACE BY MEANS OF AN OXALATING BATH CONSISTING OF OXALIC ACID INA CONCENTRATION OF ABOUT 15 GRAMS PER LITER, SODIUM NITRITE IN ACONCENTRATION OF ABOUT 1.5 GRAMS PER LITER, A FERROUS COMPOUND HAVING AFE++ CONCENTRATION OF ABOUT 0.6 GRAM PER LITER AND AN ACID STABLEAQUEOUS EMULSION OF AN ORGANIC SOLVENT HAVING A CONCENTRATION OF ABOUT15 GRAMS PER LITER, RINSING THE AMORPHOUS OXALATE LAYER SO FORMED WITHWATER AND THEN SUBJECTING SAID SURFACE TO THE ACTION OF AN AQUEOUSCHROMIC ACID SOLUTION HAVING A CHROMIC ACID CONCENTRATION BETWEEN 0.5AND 50 GRAMS PER LITER, AND THEN QUICKLY RINSING OFF ANY EXCESS CHROMICACID.